4 FM quick reads on HVAC
1. Pay Attention To Part Load Efficiency Of Heating And Cooling Distribution System
Today's tip from Building Operating Management comes from Daniel H. Nall of Flack + Kurtz. The creation of energy-efficient HVAC systems can be difficult. Many different and often conflicting factors must be optimized to achieve the best system. The prevailing climate and the function of the conditioned space are the main determinants of the most effective system. The challenge is to recognize the opportunities inherent in the climate and application so as to select the best heating and cooling sources and distribution system. One key step in the process of creating an energy-efficient HVAC system is to improve the efficiency of the heating and cooling distribution system for the building. This improvement should be thought of not only as improvement of peak-load efficiency but also of part-load efficiency, because most HVAC systems spend the preponderance of their operating life at part load.
Good part load efficiency for distribution system components often involves the use of variable speed drives along with components that allow those drives to operate at lower frequencies as often as possible. For fans and pumps, facilitating variable flow operation is a must.
For variable flow systems to be effective, capacity reduction should be accompanied by flow reduction. Two-way control valves should almost always be specified for hydronic distribution systems. Some systems require a minimum flow rate, so the use of a controlled minimum flow bypass may be required.
The bypass is preferred to the option of utilizing a limited number of three-way valves, because the three-way valves will increase flow through the system when the actual required flow is above the minimum, resulting in increased pumping energy.
Appropriate selection of the prime movers is also important for energy efficient distribution systems. Pump and fan curves can be compared to find the best selection for each application. In general, larger diameter, slow rotation speed selections are more efficient, up to a point, although the designer should avoid selections for which a slight miscalculation of the system pressure drop might result in an undesirable operating point.
2. Optimize Heating And Cooling Source Equipment To Increase HVAC Energy Efficiency
Today's tip from Building Operating Management comes from Daniel H. Nall of Flack + Kurtz. With the HVAC system's air and water distribution systems optimized, the heating and cooling source equipment can be optimized. There is a basic conflict between optimizing the efficiency of the distribution systems and optimizing the efficiency of the heating and cooling source equipment. For the distribution system, hotter hot water and colder chilled water can result in a greater temperature differential across the system, resulting in lower required flow and lower transport energy consumption. For the sources of heating and cooling, however, cooler hot water and warmer chilled water result in more energy-efficient production of these resources. Optimization of these conflicts will result in the most energy-efficient systems.
For both boilers and heat-pump-cycle heat sources, lower hot water temperature results in greater efficiency. Lower hot water temperature allows the utilization of condensing boilers and even lower hot water temperature increases the efficiency of the condensing boiler. Maximizing the efficiency of the entire system relies upon maximizing the thermal coupling between the distribution medium, air or water and the end use, the conditioned space. The key to optimal efficiency heating is thus close approach heating coils, or extended surface area convectors or radiant panels. By reducing the temperature differential between space and distribution medium (air or water), temperature differential across the transport system can be maximized while maintaining a lower-temperature heating source.
3. Data Issues Are Critical With Fault Detection And Diagnostics (FDD)
Today's tip from Building Operating Management comes from Jim Sinopoli of Smart Buildings LLC. Facility managers who are considering fault detection and diagnostics tools should be aware of the importance of data and network issues. Here are some points to keep in mind.
Lack of Data. Fault detection and diagnostics tools rely on data from building automation systems. If there are not enough sensors, if the sensors are inaccurate, or if the building has a legacy control system and for some reason accessing the BMS database or controllers is difficult, there can be issues with obtaining the accurate data required.
Using the Diagnostic Data. Many of the fault detection and diagnostics software tools can provide information to the technician or engineer regarding potential corrective actions. This information needs to be integrated into the work order system, which may be one application in a whole suite of facility management applications, in order to use the information effectively.
Applications in the Cloud. Many companies offering fault detection and diagnostics software will provide the application on the client site, but have an option to provide the application as "software as a service" (SaaS) or in "the cloud." Essentially the vendor hosts the application, and the facility manager accesses the application through a normal web browser. This can be an issue with many corporate IT departments because of the need to pierce the corporate IT firewalls and security to get to the BAS data the application needs.
Prognostics Data. While fault detection and diagnostics tools seem inherently capable of providing prognostic data — that is, it can analyze fault conditions or degradation faults and predict when a component will fail or not be able to perform correctly — very little has been developed in this area. In addition, prognostic data would allow for more proactive, condition-based maintenance, which would be a different approach for facility management organizations that are reactive and corrective.
4. Unexplored Potential in HVAC Systems
When it comes to making changes to save energy, building owners like to see low-cost or even no-cost measures. Many of these smaller fixes are relatively obvious to a third-party set of eyes with design and operational experience (perhaps hired to perform an energy audit), but may be missed by those "in the trenches."
So before you start replacing expensive equipment, work with your building engineer or maintenance technician, and conduct an audit to make sure your existing equipment works properly.
Some of the simple things you may find include:
- Variable frequency drives that were set on bypass because of a problem with the drive. We often find drives running at the full 60Hz because they have never been returned to their proper automatic action state.
- Economizer dampers in a particular fixed position because of linkage or drive motor failures.
- Outside air dampers with a 2-by-4 wedged into them to keep them open because of the same control failure as above.
- Dirty primary or secondary filters cutting down on proper airflow.
Probably the most underexplored area, though, is thorough examination of the hydronic coils, primarily the cooling coils. Cooling coils, especially those with tight fin spacings, oftentimes act as filters, trapping not only particulate but also biological masses on the fins. This not only impedes the proper volume of air getting through the coils and thus to the spaces served by the HVAC system, but also reduces the heat transfer of the system, causing the compressors to work harder and thus use more energy. And it's the biologicals that can lead to that unpleasant "dirty socks" smell. So, proper cleaning of the cooling coils can not only save energy but also improve IAQ and IEQ. Please note the emphasis on "proper."
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